1. Field of the Invention
The present invention relates to flowmeters and, in particular, to flowmeters for measuring the rate of fluid flow over a wide range.
2. Prior Art
It is known to the prior art to measure fluid flow by sensing and quantifying selected physical parameters of the flow in a given flow path. One approach to the measurement of fluid flow is that found in U.S. Pat. No. 3,321,970 to Walker, and U.S. Pat. No. 3,037,384 to Good. These patents teach the construction of flowmeters which measure the flow rate of a fluid by sensing a pressure differential across an orifice. The structures disclosed allow accommodation of different ranges of flow rates by placing different sized orifices in the main flow path.
Another type of flowmeter is based upon the principle of sensing the change in temperature of a flowing fluid caused by the addition of a fixed amount of heat. These meters typically use a bypass or secondary flow path for the measurement of the flow rate. Typical mass flowmeters of this type are disclosed in U.S. Pat. No. 2,729,976 issued to Laub, and U.S. Pat. No. 3,938,384 issued to Blair.
The basic principle of measurement of flow rate through mass flowmeters of this type is the effect of differential cooling and heating of coils which are thermally coupled to the fluid passing through the sensor. Coils of resistance wire are wrapped around a sensor tube, one in the upstream and one in the downstream positions. In the simplest case, the coils are identical, or nearly identical, and are assumed to be so for this analysis. In a common configuration, the coils are a part of a Wheatstone bridge, one side of which consists of the coils, and the other side of which consists of fixed resistors. Across the top and bottom of the bridge, i.e., impressed in series with the coils and in series with the fixed resistors which parallel the coils, is a constant current source. From the current source, current flows through the coils creating heat, which heat raises the resistance of the coils, both by an identical amount as long as no fluid flow is present. Fluid flow past the coils causes some of the heat of the upstream coil to be transferred to the fluid. At the downstream side of the coils, the heat coupled into the fluid from the upstream coil is coupled out of the fluid to the downstream coil. Thus, a difference in temperature is created between the two coils, which in turn causes a difference in the resistance of the coils. Across the arms of the bridge, the difference in resistance between the two coils creates a difference in voltage which is proportional to the flow rate through the main passage. The bridge may be balanced by heating the upstream coil by an applied current to cause the resistances to be equal, in which case, the current is the analog of flow rate.
It has long been recognized that the errors of such sensors are greatest when the fluid flow through the sensor is lowest. In by-pass sensors employing heat sensitive bridge arms, this phenomenon is a result of conduction and convection effects due to heat introduced and conducted by the sensor support structure and the surrounding atmosphere, as well as by conduction through the fluid in the sensor tube due to convection even when the fluid in the main passage is not flowing at all. At low flow rates, these effects become significant. At some threshold of fluid flow, they will predominate over the effect of heat transfer in the fluid flow itself. In the vicinity of this threshold, the sensor becomes unusable. Accordingly, to achieve the greatest accuracy in flowmeters of this type, the sensor must be operated at the upper region of its scale. Its greatest accuracy will of course be obtained at the sensor's full scale.
Because flowmeters of this type divert only a relatively minor portion of the flow through the secondary path, the volume of fluid flow available for use in sensing becomes extremely low as flow rates through the main passage become low. These sensor problems have been the subject of inventive effort in the past. U.S. Pat. No. 4,461,173, issued to Olin on July 24, 1984 teaches the improvement of by-pass type flowmeters by the addition of selectable adjustment of the ratio between the quantity of fluid flow in the main branch, and the quantity of fluid flow by-passed to the sensor. By adjustment of this ratio, operation of the sensor at a point closer to full scale may be obtained over a wider range of main branch fluid flows for a given sensor. The Olin '173 patent teaches the provision of two ranges by switching different diameters orifices in series with the main flow passage.
Another source of measurement error is a result of variation in the ratio of the sample of fluid to the main passage fluid as a result of turbulence in the flow. When the ratio changes as a result of turbulence, the fluid flow measured by the sensor is no longer an accurate reflection of the fluid flow in the main passage, regardless of accuracy with which fluid flow is measured in the sensor tube. Similarly, turbulence must be avoided in the sensor tube itself, if the measurement of heat transfer, which changes with turbulence, is to be relied upon as an accurate indication of flow rate.
U.S. Pat. No. 4,487,062, issued Dec. 11, 1984, to Olin and Korpi, discloses the design of a mass flowmeter having a sensor surrounded by a vacuum or a low-pressure, low-conductivity gas, to further reduce the effects of sensor error due to convection heat transfer from one coil to another and to the ambient by means of the atmosphere in the immediate vicinity of the coils, at low flow rates. The flowmeter is provided with a cleanable sensor tube capable of being accessed, without disassembly of the entire unit being required, without loss of the vacuum or low-pressure gas enclosure's seal integrity. The device also recognizes and provides a solution to the problem of turbulence in the main passage as a result of range adjustment devices, by providing a laminar flow element which forces flow within the main passage to be laminar even when the ratio of diameter to length of the passage is insufficient for laminar flow under ordinary conditions in the absence of the laminar flow element.
In the past, there has been no entirely satisfactory solution to the problem of adaptation of flowmeters to use over extremely wide ranges. Each meter has, heretofore been somewhat specialized as to its range. To the extent that adjustment elements have been employed, they have created a vexing problems in terms of inventory maintenance since every element must be stocked in order to achieve the full capability of flow rate range adjustment, and in terms of flexibility of adjustment as each element was uniquely tailored to its particularly specialized range of measurement.
Accordingly, a need exists for a device which can be adapted to use over a wide range of main passage flow rates, while maintaining flow laminarity which is necessary for accuracy over the entire usable scale of the meter.